Electroplating chromium and chromium alloys

ABSTRACT

An improved process for electroplating chromium and chromium alloys from a substantially aqueous electrolyte containing trivalent chromium ions as the source of chromium and containing oxyanions (preferably sulphate ions) as the major anion component. The improvement comprises incorporating into said electrolyte chlorine ions in an amount of at least 0.1 gram ions per litre or bromine ions in an amount of at least 0.05 grams ions per litre, and a nitrogen compound selected from the group consisting of ammonium compounds, hydroxylamine, hydrazine, urea, aliphatic amines and hetercyclic amines; and utilizing a carbon anode and electroplating chromium from said electrolyte with said carbon anode immersed in said electrolyte. The chlorine ion is preferred and the use of ammonium compounds is preferred.

United States Patent 1191 Crowther et al.

51 Sept. 3, 1974 1 ELECTROPLATING CHROMIUM AND CHROMIUM ALLOYS 22 Filed: Feb.2 2, 1972 21 Appl. No.: 228,083

[30] Foreign Application Priority Data Feb. 23, 1971 Great Britain 05270/71 [52] US. Cl. 204/43 R, 204/51 [51] Int. Cl. C231) 5/32, C23b 5/06 [58] Field Of Search.... 204/294, 51, 105 R, 43 R [56] References Cited UNITED STATES PATENTS 2,693,444 11/1954 Snavely et al 204/51 X 2,822,326 2/1958 Safranek 204/51 X FOREIGN PATENTS OR APPLICATIONS 7,005,999 10/1970 Netherlands 204/51 1,144,913 3/1969 Great Britain 204/51 OTHER PUBLICATIONS C, J. Broclgman, fElectrochemistry Principles and Practice, pp. 152-153, (1931). TP255 B7.

Primary Examiner--G. L. Kaplan Attorney, Agent, or Firm-Herbert I-I. Goodman [5 7] ABSTRACT An improved process for electroplating chromium and chromium alloys from a substantially aqueous electrolyte containing trivalent chromium ions as the source of chromium and containing oxyanions (preferably sulphate ions) as the major anion component. The improvement comprises incorporating into said electrolyte chlorine ions in an amount of at least 0.1 gram ions per litre or bromine ions in an amount of at least 0.05 grams ions per litre, and a nitrogen compound selected from the group consisting of ammonium compounds, hydroxylamine, hydrazine, urea, aliphatic amines and hetercyclic amines; and utilizing a carbon anode and electroplating chromium from said electrolyte with said carbon anode immersed in said electrolyte. The chlorine ion is preferred and the use of ammonium compounds is preferred.

21 Claims, No Drawings ELECTROPLATING CHROMIUM AND CHROMIUM ALLOYS Known electroplating processes employ a variety of anions in the electrolyte notable among which are halogen anions and oxygen-containing anions, such as sulphate.

With the oxygen-containing anions presently used oxygen is liberated at an inert anode. If the latter is of carbon it is attacked by the oxygen with consequent erosion. Also particles of carbon are produced which may deposit on the cathode causing faults in the plated article.

If chlorine or bromine anions are used then the free halogen is liberated at the anode in preference to oxygen. However, the halogen brings its own problems and in any case the plated surface produced is not necessarily equivalent to that obtainable using an oxygen containing anion.

The present invention provides an electroplating process employing a carbon anode and an electrolyte wherein a major proportion of the anions are oxy anions and which further comprises chlorine or bromine ions and a dissolved nitrogen-containing compound which will react with chlorine or bromine in aqueous medium to form nitrogen. By use of such an electrolyte the advantageous plating characteristics afforded by use of oxygen containinganions are substantially retained whilst the gaseous products of the electrolysis evolved in the vicinity of the anode consist wholly or in part of nitrogen instead of oxygen, thus lessening or preventing the erosion of the carbon anode.

It is thought that the operative principle in the novel process is obtaining an electrolyte in which the species having the lowest discharge potential of all anionic species present, including hydroxyl ion, is chlorine or bromine ion. These ions tend therefore to be preferentially discharged during the electrolysis, giving free halogen which then reacts with the nitrogen-containing compound to generate nitrogen. Surprisingly, however, only a small proportion of the nitrogen-containing compound and halogen ions relative to-the main oxygen containing anion need to be present in order to make a substantial change in the nature of the gas evolved near the anode from oxygen to nitrogen.

The necessary concentrations of halogen ion and nitrogen-containing compound employed in the novel process will depend upon the overall characteristics of the system rendering it vunerable to the maleficient effects of oxygen and upon the degree of the consequent impairment which can be tolerated. Clearly anodes whose physical characteristics give them a comparatively high resistance to oxygen attack may be used without disadvantage with electrolytes having lower halide ions and/or nitrogen compound concentrations than would be suitable for less resistant anodes. Also decreasing the anode size and increasing the current density in a given system will make for increased oxygen attack of the electrode. It is preferred that where chlorine ions are employed in the electrolyte they are present in a concentration of at least 0.1 gramzions per litre, with bromine the preferred minimum is 0.05 gram ions per litre although in both cases concentrations down to as little as 0.01 gram ionsper litre may have beneficial effects. The upper limits of concentration of halogen ion are not critical and in many cases will be determined by considerations of solubility, bearing in mind the need to have enough of the main oxygencontaining ion present in the electroplating system. However we have noticed that in the general case bromine ion tends to reach its maximum effectiveness at concentrations of 0.] to 0.2 gram ions per litre and chlorine ion at 1.0 to 1.5 gram ions per litre.

The concentration of nitrogen containing compound is not in general critical although it is desirably at least as great as that of the halogen ion and will normally be from one half to twice, e.g. from one to one-and-a-half times that concentration. The upper limit of effectiveness of nitrogen-containing compounds appears to be reached at concentrations of about 3 to 5 molar, e.g. 4 molar.

However, it will not normally be desirableto employ amounts of nitrogen-containing compounds greater than ten percent by weight on the water present since this may unacceptably reduce the solubilities of the metal electroplating salts which generally require the use of substantially aqueous electrolyte solutions.

A convenient way of adding a nitrogen-containing compound and a source of halogen ions is to add a halide of a nitrogen-containing base which has the abovementioned characteristic of reacting with halogen in an aqueous medium to form nitrogen. If the nitrogenous compound is not a halide salt e.g. if it is .urea, the source of halogen ions will normally be the hydrogen halide or an. alkali metal halide.

While bromides may also be usedin carrying out the invention, the halide which will normally be used is the chloride, and the subsequent description will for the sake of simplicity refer only to chlorides.

When, in accordance with the invention, a source of chlorine ions and a nitrogen-containing compound as defined above is present we believe that the anode reaction proceeds in two stages: first the evolution of chlorine and second the reaction of this chlorine with the nitrogenous compound, although some chlorine may escape in the anodic gas, especially that generated at the top of the anode which has less chance of reaction with the nitrogenous compound in the bulk of the solution. The preferred nitrogenous compounds are ammonium salts and we believe that in that case the second stage of the anode reaction is as follows:

Since the chlorine ion is substantially regenerated in the solution, only the nitrogen-containing compound needs .to be replenished. If the latter is a base, it partially neutralises the HCL generated by the reaction and thus tends to maintain the pH of the bath.

The preferred nitrogen-containing compounds are ammonium compounds such as ammonium chloride and sulphate. Also suitable are hydroxylamine, hydra zine, urea and methylamine. Other substances which may be used include aliphatic amides in general including formamide; alphatic amines in general including methylamine and heterocyclic amines including piperidine. In general it is preferred that the nitrogencontaining compounds comprise not more than five, more preferably not more than four carbon atoms per molecule.

With some nitrogen-containing compounds, e.g. urea, carbon dioxide'is evolved together with nitrogen and performs a like function in militating against oxygen attack on the anode.

tion enables carbon anodes to be substituted for soluble 5 anodes in processes where the latter are normally used. Examples of this latter type of process are the plating of zinc, nickel and copper, using metallicanodes. The use of carbon anodes would be advantageous in certain circumstances in these processes, particularly in the plating of zinc, if the problem of erosion could be overcome.

In particular the invention may be applied to conventional processes using oxygen-containing anions and a non-carbon anode together with a porous diaphragm to separate anode and cathode reactions. In operation the introduction of halogen ion and nitrogen containing compound into the electrolyte may enable the anode to be of carbon and, in' many cases, the diaphragm to be removed.

The novel process for chromium plating desirably operates at a pH below 2.5, preferably 2.3, where the halide is chloride, and below 2.6 where the halide is bromide.

The main example of plating process in which the use of carbon anodes is in practice necessary is the plating of chromium or chromium alloys from a trivalent chromium bath not using a diaphragm. This is due to the fact that if an insoluble metal anode (e.g. lead) is used it assists oxidationof the or to Cr thus spoiling the plating, while if a soluble metal anode other than of chromium is used it causes contamination. If the anode is carbon and the anion in such a bath is $0., the erosion of the carbon anode can be reduced or prevented by means of the invention.

The invention may be applied .to baths having the same constitution, apart from the halide addition, as I those hitherto used. The plating baths may include conventional electroplating additives such as brightening and levelling agents. 40

The process may find particular use in chromium plating to give a hard finish. Previous chromium plating processes being trivalent chromium ion have generally been suitable for forming only decorative finished. Preferred processes of the invention are those leading to hard chromium finishes. Hard and decorative chromium finishes are known terms of art as recognized, for example, in British Standard 4641 of 1970. A normal decorative finish might have a thickness of 20 millionths of an inch or less; a hard finish might have a thickness of from V: to 30, more usually from one to five, thousandths of an inch.

The invention is illustrated'by the following Examples:

was electrolysed in a non-compartmented cell using a nickel cathode and a carbon anode. After passage of 10 ampere hours per litre of solution, no erosion of the anode material'was-noted, and a satisfactory deposit of chromium was obtained. By way of comparison, a similar solution, omitting the ammonium chloride, was similarly electrolysed, and after passage of 10 ampere hours per litre of solution, the carbon anode was badly eroded, and particles of carbon could be filtered out of the electrolyte. The deposit of chromium contained inclusions of carbon which resulted in an unsatisfactory coating.

Example 2:

To a 100 litre electrowinning bath containing 196 grams chromium sulphate per litre (Bath A) one equiv. (53.5gms) per litre of ammonium chloride was added (Bath B). The solution was electrolysed at 40C at 10 amps/dm in a non-diaphragm cell using a carbon anode (Sdm area).and a stainless steel cathode (5dm area) spaced at 3cm from each other. After the passage of 1,000 amperehours no anode attack was noted. Nitrogen evolution was the main reaction at the anode, although a trace of chlorine could be smelt occasionally. The rate of loss of ammonium. ion was found to be close to that predicted by the equation on page 5 hereinbefore, and assuming CI; to be the primary anode product.

A similar experiment (C) using Bath A without addition of ammonium chloride showed severe anode attack after 1,000 ampere-hours, the main anodic products being carbon dioxide (from the anode) and oxygen.

A similar experiment (D) using Bath A with addition of one equiv. (66grn.) ammonium sulphate per litre instead of ammonium chloride showed severe anode attack after 1,000 ampere hours, the main anodic products being again carbon dioxide and oxygen.

A similar experiment (E) using Bath A with addition of one equiv. (58.5grn) of sodium chloride per litre insteadof ammonium, showed slight anode attack after 1,000 ampere-hours, the main anodic products being chlorine gas and oxygen. 7 I

Chromium metal of 98.8 percent purity (remainder r'nainly oxygen) was produced at 30 percent current efficiency from Bath B. The voltage required across the cell was 20 percent lower than in a similar experiment using the same solution in a cell containing a microporous diaphragm, the anolyte being 30 percent sulphuric acid and the anode being 1 percent Ag/Pb alloy.

Cost of electrical power was thus 20 percent lower than in the prior art diaphragm cell, also operated at 30 percent current efficiency.

The design of the cell was also simplified, in that microporous diaphragms were not necessary, and a separate anolyte system was not required.

EXAMPLE 3:

In a plating bath (F) made up as folows:

Chromic sulphate I96 Ammonium sulphate 264 gms. per litre Boric acid 3 of solution Dimethyl formamide articles were chromium plated in a non-diaphragm cell, using a carbon anode. After the passage of 10 amperehours per litre of solution it was noted that the deposit of chromium metal'on the articles to be plated was I patchy, containing areas of low coverage. A particle of carbon could usually be found in the centre of each such thin area. Serious anode attack was noted. The in- Chromic sulphate Ammonium sulphate 198 gms. per litre Ammonium chloride 54 of solution Boric acid 3 Dimethyl formamide 80 This solution was used to chromium plate articles in a non-diaphragm cell, using a carbon anode. After the passage of 10 ampere-hours per litre of solution, bright chromium plate equivalent to that plated from the freshly prepared solution, was still being produced. Inspection of the anode revealed no attack, and filtration of the solution showed no deposit on the filter.

EXAMPLE 4:

3. Satisfactorily bright chromium plating was produced which showed no tendency to deteriorate with time. No hexavalent chromium could be detected in the solution after passage of 10 ampere-hours of current per litre.

We claim:

1. 1n the process of electroplating chromium and chromium alloys on a cathode from a substantially aqueous acidic electrolyte'containing trivalent chromium ions as the source of chromium metal which is electroplated and containing oxyanions as the major anion component, the improvement which consists in: (A) incorporating into said electrolyte at least 0.1 gram ions per litre of halogen ion as chlorine ion or at least 0.05 gram ions per litre of halogen ion as bromine ion, and a nitrogen-containing compound selected from the group consisting of ammonium compounds, hydroxylamine, hydrazine, urea, aliphatic amines and hetercyclic amines; the concentration of said nitrogencontaining compound being at least half that of said halogen ion and not greater than 5 molar and the concentration of said halogen ion being not greater than twice that of said nitrogen-containing compound; the anionic species of said electrolyte with the lowest electrical discharge potential being said chlorine or bromine ions; (B) utilizing a carbon anode and electroplating chromium from said electrolyte with said carbon anode immersed in said electrolyte.

2. A process according to claim 1 wherein said electrolyte contains chlorine ions in a concentration of 1.0 to 1.5 gram ions per litre.

3. A process according to claim 2 wherein said oxyanions are sulphate ions.

4. A process according to claim 3 wherein the concentration of said nitrogen-containing compound in said electrolye is from one-half to twice that of said chlorine ions.

5. A process according to claim 4 wherein said nitrogen-containing compound is an ammonium'compound.

6. A process according to claim 1 wherein said electrolyte contains bromine ions in a concentration'of 0.1 to 0.2 gram ions per litre.

7. A process according to claim 6 wherein said oxyanions are sulphate ions.

8. A process according to claim 7 wherein the concentration of said nitrogen-containing compound in said electrolyte is from one-half to twice that of said bromine ions.

9. A process according to claim 8 wherein said nitrogencontaining compound is an ammonium compound.

10. A process as claimed in claim 1 wherein bromine ions are present in said electrolyte in a concentration of at least 0.05 gram ions per litre.

11. A process as claimed in claim 10 wherein bromine ions are present in said electrolyte in a concentration of from 0.05 to 0.1 gram ions per litre.

12. A process as claimed in claim 1 wherein the concentration of nitrogen-containing compound is less than 10 per cent by weight in the water present.

13. A process according to claim 1 wherein said nitrogen-containing compound is an ammonium compound.

14. A process according to claim I wherein said oxyanions are sulphate ions.

15. A process according to claim 1 wherein the concentration of said nitrogen-containing compounds in said electrolyte is from one-half to twice that of said halogen'ion.

16. A process according to claim 1 wherein the concentration of said nitrogen-containing compound in said electrolyte is from 3 to 5 molar.

17. In the process of electroplating chromium and chromium alloys on a cathode from a substantially aqueous acidic electrolyte containing trivalent chromium ions as the source of chromium which is electroplated and sulphate ions as the major anion component, the improvement which consists in: (A) incorporating into said electrolyte at least 0.1 gram ions per litre of halogen ion as chlorine ion or at least 0.05 gram ions per litre of halogen ion as bromine ion and an ammonium compound; the concentration of said ammonium compound being at least half that of said halogen ion and not greater than 5 molar and the concentration of said halogen ion being not greater than twice that of said ammonium compound, the anionic species of said electrolyte with the lowest electrical discharge potential being said halogen ions; (B) utilizing a carbon anode and electroplating chromium from said electrolyte with said carbon anode immersed in said electrolyte.

18. A process according to claim 17 wherein said halogen ion is chlorine ion and the pH of said electrolyte is below 2.3.

19. A process according to claim 18 wherein said chlorine ion is present in a concentration of from 1.0 to 1.5 gram ions per litre and said ammonium compound is present in a concentration of from 3 to 5 molar.

. 8 to 0.2 gram ions per litre and said ammonium compound is present in a concentration of from 3 to 5 molar. 

2. A process according to claim 1 wherein said electrolyte contains chlorine ions in a concentration of 1.0 to 1.5 gram ions per litre.
 3. A process according to claim 2 wherein said oxyanions are sulphate ions.
 4. A process according to claim 3 wherein the concentration of said nitrogen-containing compound in said electrolye is from one-half to twice that of said chlorine ions.
 5. A process according to claim 4 wherein said nitrogen-containing compound is an ammonium compound.
 6. A process according to claim 1 wherein said electrolyte contains bromine ions in a concentration of 0.1 to 0.2 gram ions per litre.
 7. A process according to claim 6 wherein said oxyanions are sulphate ions.
 8. A process according to claim 7 wherein the concentration of said nitrogen-containing compound in said electrolyte is from one-half to twice that of said bromine ions.
 9. A process according to claim 8 wherein said nitrogen-containing compound is an ammonium compound.
 10. A process as claimed in claim 1 wherein bromine ions are present in said electrolyte in a concentration of at least 0.05 gram ions per litre.
 11. A process as claimed in claim 10 wherein bromine ions are present in said electrolyte in a concentration of from 0.05 to 0.1 gram ions per litre.
 12. A process as claimed in claim 1 wherein the concentration of nitrogen-containing compound is less than 10 per cent by weight in the water present.
 13. A process according to claim 1 wherein said nitrogen-containing compound is an ammonium compound.
 14. A process according to claim 1 wherein said oxyanions are sulphate ions.
 15. A process according to claim 1 wherein the concentration of said nitrogen-containing compounds in said electrolyte is from one-half to twice that of said halogen ion.
 16. A process according to claim 1 wherein the concentration of said nitrogen-containing compound in said electrolyte is from 3 to 5 molar.
 17. In the process of electroplating chromium and chromium alloys on a cathode from a substantially aqueous acidic electrolyte containing trivalent chromium ions as the source of chromium which is electroplated and sulphate ions as the major anion component, the improvement which consists in: (A) incorporating into said electrolyte at least 0.1 gram ions per litre of halogen ion as chlorine ion or at least 0.05 gram ions per litre of hAlogen ion as bromine ion and an ammonium compound; the concentration of said ammonium compound being at least half that of said halogen ion and not greater than 5 molar and the concentration of said halogen ion being not greater than twice that of said ammonium compound, the anionic species of said electrolyte with the lowest electrical discharge potential being said halogen ions; (B) utilizing a carbon anode and electroplating chromium from said electrolyte with said carbon anode immersed in said electrolyte.
 18. A process according to claim 17 wherein said halogen ion is chlorine ion and the pH of said electrolyte is below 2.3.
 19. A process according to claim 18 wherein said chlorine ion is present in a concentration of from 1.0 to 1.5 gram ions per litre and said ammonium compound is present in a concentration of from 3 to 5 molar.
 20. A process according to claim 17 wherein said halogen ion is bromine ion and the pH of said electrolyte is below 2.6.
 21. A process according to claim 20 wherein said bromine ion is present in a concentration of from 0.1 to 0.2 gram ions per litre and said ammonium compound is present in a concentration of from 3 to 5 molar. 